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# Water Activity Effect on D-Value

 Author Topic Search   Topic Options Admin Admin Group Administrator Joined: 01 Sep 2009 Location: N. Ireland Status: Offline Points: 390 Post Options    Thanks(0)    Quote  Reply Topic: Water Activity Effect on D-Value    Posted: 07 May 2012 at 8:51pm Welcome to the forum. In general as the water activity (aw ) decreases (solids increase e.g. sugars and fats) in a food the heat resistance of microorganisms and their D-values increase. As a consequence more heat (and higher F values) is required to obtain a similar log reduction in microbial numbers. The opposite occurs when the water activity increases. Because the D-value generally increases (sometimes by a large factor) when aw increases, the number of log cycles that a pathogen or spoilage organism is killed will reduce significantly potentially resulting in public health or spoilage problems. I will try to explain this. Let’s say we have a heat process that has a Fo of 3 and a pathogen that has a D-value of 0.21 minutes. The Fo process will reduce the number of pathogens in the final product by 3/0.21 =14.3 log cycles. A very large reduction. If the aw is reduced e.g. by the addition of sugars, milk fat and protein the new D-value might  increase to say 0.70 minutes. It might even be higher! Since the Fo remains the same (unless we change it) the number of log cycles the pathogen will be reduced is 3/0.7 = 4.3. This would not be an acceptable process.Does this explain your query? Sponsored Links cordyceps Newbie Joined: 07 May 2012 Location: California, USA Status: Offline Points: 2 Post Options    Thanks(0)    Quote  Reply Posted: 07 May 2012 at 4:43pm How does Water Activity affect the lethality curve? Do we need to consider it in calculation of D-and F-values?(Please note that Water Activity is not merely the moisture content of the product.)Thanks!

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